Added the matrix builder contracts, along with simple test cases to excercise each of the contracts.  Additionally, added a simple implementation of MatrixBuilder for constructing complete matrices.
Also modified vector_test.rb to require the local matrix file instead of including the matrix contract in the built-in matrix itself.
Added contracts for MatrixBuilder equality, and fixed a bug in the == method of CompleteMatrixBuilder.
Added some tests for the == method of MatrixBuilder.
Made MatrixBuilder enumerable for ease of implementation and testing.
Merge pull request #17 from isupeene/builder_contracts

Added the matrix builder contracts...